Atypia of Undetermined Significance (AUS) in Thyroid FNA. Malignancy Rate and Value of Substratification
Hangjun Wang, Allen S Ho, Luc GT Morris, Oscar Lin. Memorial Sloan-Kettering Cancer Center, New York, NY
Background: The Bethesda System for reporting thyroid cytopathology (TBSRTC) includes a category of Atypia of Undetermined Significance (AUS). The AUS category has an implied risk of malignancy between 5-15%according to TBSRTC, although several studies have shown a higher rate of malignancy, ranging from 20 to 40 %. Our objective was to analyze the rate of malignancy in thyroidectomies that had a previous diagnosis of AUS at a comprehensive cancer center, where the rate of AUS diagnosis is 8%, close to the recommended rate by TBSRTC. We also proposed to evaluate the role of substratifying the diagnosis of AUS.
Design: Patients with a thyroid FNA diagnosis of AUS who underwent thyroidectomy between 2007-2011 were selected for this study.The diagnosis was further stratified into 5 categories as following: AUS, NOS; AUS favors benign; AUS cannot rule out papillary thyroid carcinoma (PTC); AUS cannot rule out Hurthle cell neoplasm; and AUS cannot rule out follicular neoplasm. The cytologic diagnoses were correlated with findings in the thyroidectomy specimens to determine the benign (goiter and lymphocytic thyroiditis), neoplastic (including adenomas) and malignancy rate.
Results: The findings in 139 patients who underwent thyroidectomies following a diagnosis of AUS were analyzed. The rate of malignancy in the resected nodules matching the AUS diagnosis was 37% overall, however an additional 23% of patients also had an incidental microcarcinoma not sampled by FNA. Upon stratification of the AUS category as proposed above, the malignancy rate was significantly higher (p<0.001) if a specific neoplasm could not be ruled out when compared to the AUS, NOS or AUS, favor benign. A summary of the malignancy rate in each AUS subcategory is listed in table 1.
|FNA/thyroidectomy (number of cases)||Benign (%)||Neoplasm (%)||Malignant (%)|
|AUS only,NOS (65)||60||40||25|
|AUS, favor benign (13)||77||23||8|
|AUS,cannot r/o PTC (13)||23||77||54|
|AUS, cannot r/o Hurthle cell neoplasm (15)||26||74||47|
|AUS, cannot r/o follicular neoplasm (33)||18||82||59|